Effect of Temperature and Time on Zinc Borate Species Formed from Zinc Oxideand Boric Acid in Aqueous Medium

H. Emre Eltepe,† Devrim Balko1se,*,‡ and Semra U2 lku1‡

Pigment AS¸ Kemalpas¸a, Izmir, Turkey, and Department of Chemical Engineering, Izmir Institute of Technology,Gulbahce Koyu, Urla-Izmir 35430, Turkey

The effect of temperature and time of heating of zinc oxide and boric acid in aqueous medium on producttype, dehydration behavior, crystal morphology, and structure was investigated for the production of flameretardant and smoke suppressant zinc borate. Two different products dehydrated at 140 and 350°C wereobtained and characterized by thermal gravimetric analysis, X-ray diffraction, energy dispersive spectroscopy,and Fourier transform infrared spectroscopy.

1. Introduction

Plastic materials widely used all throughout our lives releasesmoke and toxic gases during heating at high temperatures. Zincborates (ZBs) are widely used flame retardants and smokesuppressants. Zinc borates have been used in flame retardingof paints1 and EVA.2 Giudice and Benitez1 discussed theinfluence of ZB with molecular formulas containing 3.5 H2Oand 7.5 H2O on flame resistance of paints. Eight different paintcompositions were prepared. Limiting oxygen index (LOI)according to ASTM D 2863 and flame spread index (FSI) testswere carried out to test the flame retardancy. The compositionshaving both ZBs and having only 7.5 H2O with antimonytrioxide had the maximum LOI values. The increase in the useof synergistic mixture of zinc borate together with aluminumtrihydrate has also increased flame retardency. The reason forthis usage is that the synergistic mixture of these two compoundspromotes a nonhalogen char formation.1 Bourbigot et al.2 havestudied the recent advances in the use of zinc borate in the flameretardancy of EVA. In their work, zinc borates were used assynergistic agents in ethyl vinyl acetate-aluminum trihydrate(EVA-ATH) and EVA-Mg(OH)2 flame retardant formulationsand as smoke suppressants. The flame retardancy of low-densitypolyethylene (LDPE) treated with complex flame retardantcomposed of ultrafine zinc borate (UZB) and intumescent flameretardant (IFR) was maximum when the mass ratio of UZB toIFR is 4.2:25.8, and the complex flame retardant mass contentwas 30% (based on LDPE). Boron orthophosphate (BPO4) andzinc-containing compounds were formed in the residual char,and these substances may play an important role in stabilizingthe intumescent char structure and decreasing the degradationspeed substantially when subjected to high temperature.3 Thestudy about the effects of zinc borate (ZB), aluminum trihydrate(ATH), and their mixture on the flame-retardant and smoke-suppressant properties of poly(vinyl chloride) (PVC) shows thatthe incorporation of a small amount of ZB, ATH, and theirmixture can greatly increase the LOI of PVC and reduce thesmoke density of PVC during combustion. The mixture of ZBwith ATH has a good synergistic effect on the flame retardanceand smoke suppression of PVC. Thermal gravimetric analysis

(TGA) and gas chromatography-mass spectroscopy (GC-MS)analysis results show that the incorporation of a small amountof ZB, ATH, and their mixture greatly promotes the charformation of PVC and decreases the amount of hazardous gasessuch as benzene and toluene released in PVC during combustion.Zinc borate as a flame retardant formulation on some tropicalwoods reduced drastically their flame propagation rate, afterglow time, and flame temperature and increased their residue(char) and LOI. It was observed that zinc borate acidified withHCI functions as a flame retardant (FR) formulation by acomplex process that entails the dehydration and condensedphase and vapor phase mechanisms.5

A series of different crystal structures of zinc borate (ZB)have been developed and are being used. The most widely usedones are zinc borate with the formula 3ZnO‚2B2O3‚3.5H2O,2ZnO‚3B2O3‚3H2O, and anhydrous ZB 2ZnO‚3B2O3. The heatstability of zinc borate is between 290-300 °C which enablesthe polymer processibility and the heat stability of anhydrousborate up to 400°C. Zinc borate 2ZnO‚3B2O3‚7H2O is formedwhen borax is added to aqueous solutions of soluble zinc saltsat temperatures below about 70°C. An X-ray structure deter-mination has indicated that this compound is orthorhombic andhas a zinc triborate monohydrate structure Zn[B3O3(OH)5]‚H2O.Zinc borates 2ZnO‚3B2O3‚7H2O and ZnO‚B2O3‚2H2O releasewater when heated from 130 to 250°C.6

Zinc borate (2ZnO‚3B2O3‚3.5H2O) in general is producedwith the reaction between zinc oxide and boric acid. Boric acidis dissolved in water between the temperatures 95 and 98°C,and zinc oxide and seed crystals of 2ZnO‚3B2O3‚3.5H2O isadded to this solution at a certain stoichiometric ratio. Thereaction continues for a while by mixing, and the zinc borateformed is filtered, dried, and ground. Schubert et al.7 found outthat 2ZnO‚3B2O3‚3.5H2O was actually Zn[B3O4(OH)3] (ZnO‚3B2O3‚3H2O written in oxide form). The structure of Zn[B3O4-(OH)3] was determined for the first time by single-crystal X-raydiffraction and H magic angle spinning (MAS) NMR, revealingit to be a complex network consisting of infinite polytriboratechains crosslinked by coordination with zinc and furtherintegrated by hydrogen bonding. The boroxyl oxygen involvedin zinc coordination lies between the BO3 and BO4 polyhedrainvolved in chain extension. The remaining two zinc coordina-tion sites are occupied by hydroxyl oxygens of the BO2(OH)2groups in two separate polyborate chains. In this way, allhydroxyl groups are involved in zinc coordination. 2ZnO‚3B2O3‚3H2O hydrolyzes to ZnO and boric acid in aqueous medium if

* To whom correspondence should be addressed. Prof. Dr. DevrimBalkose, Faculty of Engineering, Department of Chemical Engineering,Gulbahcekoyu Urla Izmir, Turkey. Tel.: +90 232 750 6642. Fax:+90232 750 6645. E-mail address: devrimbalkose@iyte.edu.tr.

† Pigment ASKemalpas¸a.‡ Izmir Institute of Technology.

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10.1021/ie0610243 CCC: $37.00 © 2007 American Chemical SocietyPublished on Web 03/16/2007

the temperature exceeds 90°C. Further in the article, the correctformula of 2ZnO‚3B2O3‚3.5H2O will be used as 2ZnO‚3B2O3‚3H2O.

Nies et al.8 developed a method for producing zinc boratewith a low hydration number by using sodium borate and zincsalts.

In another patent by Igarashi et al.,9 zinc borate having aparticular crystallite size and containing very little sodiumcomponents and a method of preparing the same was studied.The zinc borate had a particular chemical composition, had acrystallite size not smaller than 40 nm as found from diffractionpeaks of indexes of planes of (020), (101), and (200) in theX-ray diffraction image (Cu KR), and contained sodiumcomponents in amounts not larger than 100 ppm as measuredby the atomic absorption spectroscopy. Igarashi et al.9 prepared2ZnO‚3B2O3‚3.0H2O from boric acid and zinc oxide in twosteps. In the first step at 60°C for 1.5 h, zinc borate crystalsform, and in the second step at 90°C for 4 h, the crystal growthoccurs. Schubert et al.7 indicated that the formation of 2ZnO‚3B2O3‚7H2O was initially from ZnO and boric acid and that itis transformed to 2ZnO‚3B2O3‚3H2O by slow polymerizationof borate ions in the solid state.

In a US patent by Schubert,10 zinc borate compositions havinga ZnO:B2O3 ratio of 4:1 and anhydrous zinc borate wereexamined. The patent relates to improved zinc borate composi-tions and, more particularly, provides a new hydrated zinc boratehaving a high dehydration temperature which offers significantadvantages for compounding with plastics and rubbers atelevated temperatures. The anhydrous form of zinc borate wasalso provided in this study, offering advantages for compoundingat even high temperatures.

Shete et al.11 have studied the kinetics of the fluid-solidreaction of zinc borate by the reaction between zinc oxide andboric acid. Mixing parameters influencing the final particle sizeand conversion of zinc oxide were studied for the formation ofzinc borate. The formation of zinc borate is via a fluid-solidreaction. The process was kinetically controlled above theminimum speed for particle suspension. The reaction kineticswas developed, and the rate constant was estimated in this study.The reaction was assumed to be a first-order surface reactionwith respect to boric acid, and rate constants were found fromexperimental conversion time data. Shete et al.11 considered thediffusion of boric acid to ZnO particles but not the rearrange-ment of borate rings with time to polyborate ions in their kineticmodel.

Thousands of metric tons of this material have been manu-factured by several companies around the world for more than35 years using the reaction of zinc oxide with boric acid.Although zinc borate production has long been a widely knownand practiced technology, there is no study of the characteriza-tion of intermediates formed. Thus, the effects of time andtemperature of heating, seed crystals, and refluxing on the typeand properties of zinc borate species obtained from zinc oxideand boric acid in aqueous medium were the aim of investigationin this work.

2. Experimental

2.1. Materials. The boric acid supplied by ETI˙BANK witha molecular weight 61.83 and a molecular formula of B(OH)3

with 99.9% purity and the zinc oxide (99%) from Ege Kimya

were used throughout the experiments. Commercial zinc boratescalled ZB-2335 with 37% ZnO and 47% B2O3 and a maximumof 16% water of crystallization and Chinese zinc borate with37.5% ZnO and 48% B2O3 and with ignition loss at 400°C asa maximum of 14.5% were used in the experiments as referencematerials.

2.1. Methods.The synthesis of 2ZnO‚3B2O3‚3H2O was doneaccording to the overall reaction given in eq 1.

The experiments were carried out by first dissolving therequired amount of boric acid in a separate beaker in pure water.Since the solubility of boric acid at room temperature (20°C)is 5.04%, the solution of 290 g/dm3 boric acid in water wasprepared by heating the mixture at 60°C. After boric acid wascompletely dissolved in water, zinc oxide was added to makethe concentration 96 g/dm3 ZnO in the mixture making the molarmolar ratio of B2O3 to ZnO be 2. An excess amount of boricacid was used to ensure the formation of 2ZnO‚3B2O3‚3.0H2O.4

Table 1. pH Change after Mixing ZnO and Boric Acid at 60 °C

time (s) 0 10 30 60 120 180 240 360 420pH 4.30 5.33 5.63 5.68 5.70 5.72 5.72 5.72 5.72

Figure 1. TGA curves of commercial ZB: (1) ZB-2335; (2) Chinese ZB.

Figure 2. TGA curves of samples prepared at 60°C for 90 min (1) withseed crystal and (2) without seed crystal.

Figure 3. TGA curves of product formed at 90°C for 4 h (1) with seedcrystal and (2) without seed crystal (they were heated at 60°C for 1.5 hinitially).

2ZnO(s)+ 6H3BO3(aq)f

2ZnO‚3B2O3‚3H2O(s)+ 6H2O (1)

2368 Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

A 0.125 g portion of ZB 2335 was added as seed crystal. Thismixture in closed Schott bottles was stirred for 1.5 h at thistemperature in a constant temperature bath. After a smallsample was drawn, the mixture was further stirred and reactedat 90°C for 4 h. Mixing was achieved by shaking the bottlesin a temperature controlled water bath at a rate of 170 rpm.The pH of the mixture was measured by a Methrom pH meterwith a glass electrode from time to time. A second experimentunder the same conditions but without any seed crystal was alsodone. The samples were filtered using a Buchner funnel anddried at 105°C for 2 h in an aircirculating oven.

2.2. Characterizations.Each sample was characterized byX-ray diffraction, Fourier transform infrared spectroscopy(FTIR), scanning electron microscopy (SEM), energy dispersiveX-ray (EDX) analysis, and thermal gravimetric analysis (TGA).Microstructural characterization of the samples was done by

scanning electron microscopy using a Philips XL-305 FEG SEMinstrument. The elemental composition of the samples weredetermined by conducting energy dispersive X-ray (EDX)analysis in the same instrument.

All of the samples were characterized by X-ray diffractometer(Philips Xpert-Pro) to analyze the crystal structures of the ZBwith Cu KR radiation at 45 kV and 40 mA. The registrationswere performed in the 5°-60° 2θ range.

Thermal gravimetric analyses (TGAs) were done with aShimadzu TGA-51. ZB samples (10-15 mg) were loaded intoan alumina pan and heated from 30 to 600°C at 10°C/minunder N2 flow (40 mL/min).

To determine which functional groups are present in thesamples, a Shimadzu FTIR 8601 was used. The KBr discmethod was used by mixing 4.0 mg of ZB and 196 mg of KBr.A pellet was obtained by pressing the powder mixture under 8tons of pressure.

3. Results and Discussion

3.1. Reactions.The following reactions are expected to occurin zinc borate preparation. Solid boric acid dissolution (eq 2),hydration of ZnO in water (eq 3), formation of Zn[B3O3(OH)5]‚H2O(s) at 60°C (eq 4), and formation of Zn[B3O4(OH)3](s) at90 °C (eq 5).

Figure 4. SEM microphotographs of commercial ZB and raw materials(a) ZB-2335 (25 000×), (b) Chinese ZB (25 000×), (c) Boric acid (1000×),and (d) Zinc oxide (25 000×).

Figure 5. SEM microphotographs of samples prepared at 60°C for 1.5 h(a) with seed crystals (8000×) and (b) without seed crystals (10 000×)and of samples prepared at 90°C for 4 h (they were heated at 60°C for 1.5h) (c) with seed crystals (8000×) and (d) without seed crystals (15 000×).

Figure 6. FTIR spectra of (1) ZB-2335 and (2) sample prepared at 60°Cfor 1.5 h without seed crystal.

Figure 7. FTIR spectra of (1) ZB-2335 and (2) sample heated at 90°Cfor 4 h (initially, it was heated at 60°C for 1.5 h).

Table 2. Properties of Reference Materials and ProductsDetermined by TGA and EDX

sampleH2O loss at600°C (%)

dehydronsetT (°C)

B2O3/ZnO(molar ratio)

60 °C for 1.5 hwith seed crystal

21 130 1.49

60 °C for 1.5 h and 90°Cfor 4 h with seed crystal

10 350 1.40

60 °C for 1.5 h withoutseed crystal

27 131 1.78

60 °C for 1.5 h and 90°Cfor 4 h without seed crystal

12 340 1.68

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Since an excess of boric acid was used, the solution pH isexpected to be acidic. In the experimental run to follow pHchange during reaction, it was found that the pH of the boricacid solution was 4.3 and there was a step change to 5.33 in 10s after ZnO was added as reported in Table 1. The pH wasstabilized at 5.72 after 180 s, and it did not change with timeany more. Thus, during preparation of ZB, the pH was lowerthan a pH of 7 and the reaction medium was acidic as expected.This showed that the neutralization occurred instantly after theaddition of ZnO to boric acid as there was a step change to pH5.33 in 10 s. The formed product was not the desired crystalmodification of ZB with 3 mol water, but it was a form with ahigher amount of crystal water. The further mixing of ZnO andboric acid at 90°C for 4 h4,6,7 under reflux would result in aZB formation which has the desired chemical formula, 2ZnO‚3B2O3‚3 H2O, and thus the desired dehydration temperature.

3.2. Mixing Problems.The volume of the solid products inthe reaction mixture increases with time, and mixing problemsoccur due to an increase in viscosity. The specific gravities ofZnO and 2ZnO‚3B2O3‚3H2O are 5.3 and 2.77 g/cm3, respec-tively. In the patent by Igarashi et al.,9 1 L of water contained290 g of boric acid and 96 g of ZnO while boric acid is presentin dissolved form and ZnO is a solid. The B2O3/ZnO ratio was2.0 in the reactant mixture, but in the product, it was 1.5. Themass of the solid fraction increases 2.66 times and correspondsto a volume fraction increase of 5.11 times considering thedensities. This causes a 4.36-fold increase in the viscosity ofthe mixture during the beginning and end of the reaction asfound using eq 6.

whereη is viscosity of the suspension,ηs is the viscosity of thesolvent,kE is 2.5 for spherical particles, andφ is the volumefraction of the dispersed solid phase.

Thus, a mechanical mixer which will stir the mixture evenwhen the viscosity is high should be used for this reaction. Inthe present study, a temperature controlled water bath was usedsince a small amount of reactants was in consideration. By usinga closed container, water loss by evaporation was prevented.In large scale production, a reactor under reflux and with a verypowerful mechanical mixer should be used.

3.3. Characterizations.The physicochemical properties ofeach sample as well as the reference ZBs were determined bythermal gravimetric analysis (TGA), Fourier transform infraredspectroscopy (FTIR), scanning electron microscopy (SEM),energy dispersive X-ray (EDX) analysis, and X-ray diffraction.

The upward slope of the TGA curves in Figures 1-3 duringthe initial periods of heating was due to a small drift in thebaseline of the instrument. TGA curves of commercial zincborates in Figure 1 indicated that they were very stable up to350 °C and they lost 8% and 6% of their mass for ZB 2355and China ZB, respectively, up to 600°C. The 2ZnO‚3B2O3‚3H2O has been reported to be thermally stable up to 290°C by

Schubert at al.7 The dynamic heating program of the TGAanalysis should have caused to a shift to higher temperaturesin observation of the thermal stability temperature. TGA curvesof samples prepared at 60°C for 1.5 h without and with seedcrystals are given in Figure 2. For samples prepared at 60°Cfor 1.5 h with seed crystals, ZB starts to lose its hydration waterat 130°C with a mass loss of 21% until 600°C, and for samplesprepared at 60°C for 1.5 h without seed crystals, ZB starts tolose its mass at 131°C with a mass loss of 26% until 600°C.When these two samples are compared, it is observed that theone with seed crystals has less mass loss due to dehydrationthan that without seed crystals. For samples heated further at90 °C for 4 h, the mass losses started at 350 and 340°C andamounts to 12% and 10% at 600°C for the samples with andwithout seed crystals, respectively, as seen in Figure 3. Thesesamples had dehydration temperatures and mass losses very

B(OH)3(s)f B(OH)3(aq) (2)

ZnO(s)+ H2O f Zn(OH)2(s) (3)

Zn(OH)2(s) + 3B(OH)3(aq)f

Zn[B3O3(OH)5]‚H2O(s)+ 2H2O (4)

Zn[B3O3(OH)5]‚H2O(s)f

Zn[B3O4(OH)3](s) + 2H2O (5)

η ) ηs(1 + kEφ) (6)

Figure 8. X-ray diffraction diagrams of reference ZB materials and rawmaterials.

Figure 9. X-ray diffraction diagram of sample prepared at 60°C for 1.5h without seed crystal.

Figure 10. X-ray diffraction diagrams of sample heated at 90°C for 4 hwithout seed crystal (initially, it was heated at 60°C for 1.5 h).

2370 Ind. Eng. Chem. Res., Vol. 46, No. 8, 2007

close to commercial ones. On the other hand, if 2ZnO‚3B2O3‚3H2O were obtained, mass loss should have been 14.9%. Thesample prepared without seed crystals at 90°C had as a closermass loss (12%) to that of 2ZnO‚3B2O3‚3H2O. The mass lossvalues at 600°C are reported in Table 2.

The SEM micrograph of zinc oxide particles in Figure 4shows that they were in needle shape and were a maximum of2 µm long. Two commercial ZBs had similar morphologies asseen in Figure 4. SEM microphotographs of samples preparedat 60 °C for 1.5 h with and without seed crystals were verysimilar to each other as seen in Figure 5. There were nounreacted ZnO crystals with needle shape. The samples furtherheated at 90°C for 4 h had a similar morphology to those whichare used as reference products as seen in Figures 4 and 5.

The FTIR spectrum of the sample prepared at 60°C for 1.5h was different than the FTIR spectrum of commercial ZB 2335as seen in Figure 6. While the characteristic peak at 3227 cm-1

of ZnO‚B2O3 crystal structure2 is not observed in the sample’sspectrum, ZB 2335 had two sharp peaks at 3250 and 3500 cm-1

belonging to vibrations of isolated OH groups.9 In Figure 7, itis seen that the sample prepared by heating at 90°C for 4 hhad the same FTIR spectrum as commercial ZB 2335. The peaksbetween 900 and 1300 cm-1 are all present in the spectra whichwere reported to be other characteristic peaks of borates.12 Thecharacteristic peaks of tetrahedral (BO4) and trihedral (BO3)borate groups are clearly seen between 1100 and 800 cm-1 andbetween 1450 and 1200 cm-1.

The chemical compositions of the samples were determinedby EDX analysis as reported in Table 2. While the theoreticalB2O3/ZnO ratio was 1.50 for 2ZnO‚3B2O3‚3H2O, it was 1.40for samples prepared with seed crystals and it increased to 1.78and 1.68 for samples prepared without seed crystals. Thepresence of seed crystals caused the formation of products closerto the B2O3/ZnO ratio in 2ZnO‚3B2O3‚3H2O.

X-ray diffraction diagrams of raw materials and commercialZBs are seen in Figure 8. Boric acid and ZnO had X-raydiffraction diagrams of pure materials as indicated by the X-pertpro software. Commercial ZBs had the same X-ray diffractiondiagram of 2ZnO‚3B2O3‚3H2O reported by Igarashi et al.9 andNies et al.8 The X-ray diffraction diagrams of the samplesprepared at 60°C with and without seed crystals were the same.In Figure 9, the X-ray diffraction diagram of a sample preparedat 60 °C is shown and it has no similarity to the ZB X-raydiffraction diagram reported by Igarashi et al.9 A strongdiffraction peak at a 2θ value of 8° corresponding to 1.11 nmpresent in the sample prepared at 60°C was not present in theX-ray diffraction diagram of 2ZnO‚3B2O3‚3H2O samples pre-

pared at 90°C for 4 h. The X-ray diffraction diagrams of thesamples prepared at 90°C in Figure 10 were identical to eachother and to that of 2ZnO‚3B2O3‚3H2O reported by Igarishi etal.9 X-ray diffraction peak intensities of the samples preparedat 60 and 90°C are as shown in Table 3.

4. Conclusions

Using a powerful mixer and good temperature control andkeeping the volume constant by using reflux to prevent waterevaporation are important requirements in obtaining the desiredzinc borate species, since the viscosity of the medium increasesto a great extent during reaction.

Two different zinc borates dehydrating at around 130°C with21-27% mass loss and at around 350°C with 10-12% massloss were obtained at 60°C and further heating at 90°C,respectively, from zinc oxide and boric acid in water. Theyshould be Zn[B3O3(OH)5]‚H2O or 2ZnO‚3B2O3‚7H2O and Zn-[B3O4(OH)3] or 2ZnO‚3B2O3‚3H2O. The X-ray diffractiondiagrams of the samples prepared at 90°C with and withoutseed crystals were identical to that of 2ZnO‚3B2O3‚3H2Oreported by Igarishi et al.,9 but their B2O3/ZnO ratios weredifferent than that of 2ZnO‚3B2O3‚3H2O. Their TGA curvesand FTIR spectra were the same as the two commercial zincborates which are claimed to be 2ZnO‚3B2O3‚3H2O by theirproducers.

Literature Cited

(1) Giudice, C. A.; Benitez, J. C. Zinc Borates As Flame-retardantPigments In Chlorine containing Coatings.Prog. Org. Coat.200142, 82.

(2) Bourbigot, S.; Bras, M. L.; Leeuwendal, R.; Shen, K. K.; Schubert,D. Recent Advances In the Use of Zinc Borates In Flame Retardancy ofEVA. Polym. Degrad. Stab.1999, 64, 419.

(3) Wu, Z.; Shu, W.; Hu, Y. Synergist Flame Retarding Effect ofUltrafine Zinc Borate on LDPE/IFR System.Appl. Polym. Sci. 2000, 103,3667.

(4) Ning, Y.; Gua, S. Flame-Retardant and Smoke-Suppressant Propertiesof Zinc Borate and Aluminum Trihydrate-Filled Rigid PVC.J. Appl. Polym.Sci.2000, 77, 3119.

(5) Garba, B. Effect of zinc borate as flame retardant formulation onsome tropical woods.Polym. Degrad. Stab. 1998, 40.

(6) Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; John,Wiley and Sons: New York, 1994; Vols. 10 and 4.

(7) Schubert, D. M.; Alam, F.; Mandana, Z. V.; Knobler, C. StructuralCharacterization and Chemistry of The Industrially Important Zinc Borate,Zn[B3O4(OH)3]. Chem. Mater.2003, 15, 866.

(8) Nies, N. P.; Beach, L.; Hulbert, R. W. Zinc Borate with low hydrationand method for preparing the same. US Patent No. 3,649,172, 1972.

(9) Igarashi, H.; Tatebe, A.; Sakao, K. Zinc Borate, and ProductionMethod and Use Thereof. EP Patent No. 1,205,439A1, 2001.

(10) Schubert, D. M. Zinc Borate. US Patent No. 5,472,644, 1995.(11) Shete, A. V.; Sawant, S. B.; Pangarkar, V. G. Kinetics of Fluid-

Solid Reaction With An Insoluble Product: Zinc Borate By The Reactionof Boric Acid and Zinc Oxide.J. Chem. Technol. Biotechnol.2003, 79,526.

(12) Yongzhong, J.; Shiyang, G.; Shuping, X.; Jun, L. FT-IR spectros-copy of supersaturated aqueous solutions of magnesium borate.Spectrochim.Acta Part A1999, 56, 1291

(13) Xie, R.; Qu, B.; Hu, K. Dynamic FTIR Studies of Thermo Oxidationof Expandable Graphite-based Halogen-free Flame Retardant LLDPEBlends.Polym. Degrad. Stab.2001, 72, 313.

ReceiVed for reView August 4, 2006ReVised manuscript receiVed February 2, 2007

AcceptedFebruary 7, 2007

IE0610243

Table 3. X-ray Diffraction Peaks of the Samples

60 °C, 1.5 h 90°C, 4 h

with seedcrystals

without seedcrystals

with seedcrystals

without seedcrystals

2θ 2θ 2θ 2θ8.0 8.0

17.5 17.6 17.7 17.920.8 20.4 20.3 20.323.0 22.8 21.4 21.623.5 23.3 23.4 23.527.4 24.0 25.5 25.528.7 27.0 27.2 27.2

28.1 28.4 28.5

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